Frequency anomaly of groundwater level before major earthquakes in Taiwan

T.-K. Yeh, C.-H. Chen, C.-H. Wang, S. Wen
2015 Proceedings of the International Association of Hydrological Sciences  
<p><strong>Abstract.</strong> Anomalous decreases on water levels were observed in 78 % of wells in central Taiwan about 250 days before the Chi-Chi earthquake whose magnitude is 7.6 on 20 September 1999. Variations in groundwater levels measured on anomalous wells from 1 August 1997 to 19 September 1999, were transferred into the frequency domain to unveil frequency characteristics. Analytical results show that amplitudes at the frequency band between 0.02 and 0.04 day<sup>&amp;minus;1</sup>
more » ... amp;minus;1</sup> generally maintained at low stage and were apparently enhanced a few weeks before the Chi-Chi earthquake. Variations of amplitude at this particular frequency band were further examined along with other Taiwan earthquakes whose magnitude is larger than 6 from 1 August 1997 to 31 December 2009. Features of the enhanced amplitudes at the frequency band are consistently observed prior to the other two earthquakes during the 12.5-year study period. Result confirms that abnormal rise and fall changes in groundwater level yield an agreement with forward and backward surface displacements around epicenter prior to the Chi-Chi earthquake.</p>
doi:10.5194/piahs-372-101-2015 fatcat:hvbxhlszvfh7zoove5x7gzaetm